The present disclosure relates to an analog-digital converter and an image sensor system having the same, and more particularly to an integration technique for an analog-digital converter.
In a complementary metal-oxide semiconductor (CMOS) image sensor system, an analog-digital converter (e.g., a column ADC) including a plurality of ADC sections corresponding to a plurality of pixel columns of the image sensor is used. An ADC section converts a pixel voltage from a pixel column corresponding to the ADC section to a digital value, and includes, for example, a comparator, a counter, a digital memory, etc.
In such an image sensor system, hundreds or thousands of ADC sections corresponding to pixel columns are arranged with a certain cell pitch. Therefore, an ADC section needs to be formed within the cell pitch. However, with the increase in the demand for reducing the size of an image sensor and increasing the number of pixels thereof in recent years, the cell pitch has become very small, i.e., some μm or less, and there arises a problem that the maximum value of the size of the transistor used in the ADC section is restricted by the cell pitch.
As digital single-lens reflex cameras which are required to have a high speed and a high precision, monitoring/on-vehicle cameras which are required to have a high-sensitivity image even at night, and the like, become widespread, there is a demand for realizing high sensitivity and low noise for an image sensor system. In order to realize these, it is necessary to improve the performance of an analog-digital converter (e.g., increasing the resistance against power supply voltage fluctuations, reducing device variations, reducing noise, etc.), and it is becoming important, as a measure for this, to extend the channel length of a transistor used in an image sensor system (particularly, the ADC section).
As described above, for analog-digital converters, there is a demand for increasing the degree of integration and improving the performance. Note that such an analog converter is applicable not only to image sensor systems but also to other technical fields (e.g., panel drivers such as liquid crystal drivers and PDP drivers).
Japanese Laid-Open Patent Publication No. 2005-217158 (Patent Document 1) is known in the art as an approach to meeting the demands above. In a photoelectric conversion device of Patent Document 1, a plurality of amplification circuits including constant current circuits are arranged in a predetermined repetition direction, and the constant current circuit includes a field effect transistor. The channel length direction of the field effect transistor matches a direction perpendicular to the repetition direction of the amplification circuits. With such a configuration, the channel length of the field effect transistor is extended without being restricted by the cell pitch.